The total nitrogen content of plant tissue may be determined by direct nesslerization of sulphuric acid-hydrogen peroxide digests with accuracy up t o 5%. Similar application to soil extracts involves somewhat greater errors.In a spectrophotometric study of the Nessler colour, a filter showing maximum transmission a t 420 mp absorbs the greatest proportion of the colour.In the sulphuric acid-hydrogen peroxide digestion of plant tissue, added ammonia is fully recovered but added nitrate suffers a loss of 24-47y0, the percentage loss decreasing as the amount of nitrate present increases.Bromothymol blue is a suitable internal indicator for the neutralization of test solutions before nesslerization. The blue colour of the indicator does not affect the absorptiometric measurement of the Nessler colour.Soil solutions and soil extracts may be decolorized satisfactorily by a specially treated activated charcoal. This charcoal does not absorb appreciable amounts of ammonia from soil extracts made with Morgan's reagent, 2.5% acetic acid or water. For more accurate determinations of ammonia in soil extracts etc., distillation of the ammonia into acid in the presence of bromothymol blue, followed by nesslerization, is preferable.A modified Nessler reagent that showed diminished susceptibility to interfering factors and could be used immediately after preparation has been described in a previous paper.l The present communication deals with the use of this reagent in the micro-determination of total nitrogen in plant tissue and of ammonia-nitrogen in soil extracts and soil solutions by direct nesslerization. Some observations on the effective use of light filters in the spectrophotometric measurement of the Nessler colour are included. Spectrophotometric measurement of Nessler colourIn previous work it was shown that for the measurement of the Nessler colour a dark-blue filter OBI (approximate maximum transmission at 435 mp) afforded about three times the sensitivity of the light-blue filter OBz (480 mp).1 A Unicam spectrophotometer, which has J. Sci. Food Agric., 5, August, 1954 * Part I : J. Sci. Fd Agric., 1952, 3 , 441 YUEN G POLLARD-DETERMINATION OF NITROGEN. 11 365 FIG. I .-Spectraphotometric study of Nessler colour since become available in this Laboratory, made possible a more detailed examination of the character of the Nessler colour with respect to the visible spectrum. To each of a series of 50-ml. graduated flasks, containing respectively 0, 20, 50, IOO or 150 pg. of ammonia-nitrogen, were added I ml. of sodium potassium tartrate solution and 2 ml. of Nessler reagen't.1 The readings at different wavelengths, recorded with the Unicam spectrophotometer, are shown in Fig. I.It was evident that the spectrophotometer readings for the Nessler colour increased steadily with decrease of wavelength from 600 to 400 mp. With light transmissions between 600 and 700 mp, the absorption of Nessler colour was apparently small. Owing to the high ' blank ' obtained with wavelengths approaching 400 mp, the absolute maximum readings...
1. A modified Nessler reagent in which the ratio HgI2 : KI : NaOH is 1 : 2.2 : 20 is described. It is clear and ready for immediate use and is less susceptible to interference by various cations. Distillation with phosphoric acid was satisfactory for preparing ammonia‐free water. The best range of ammonia‐nitrogen for this determination was from 0 to 100 μg. 2. The amount of the reagent used, the time and temperature of colour development, and the acidity of the test solution were critical factors in the accuracy of the determination. 3. Many cations interfered with the test to various extents. Among anions tested only arsenite, chromate, cyanide, dichromate and permanganate caused serious interference. Addition of sodium potassium tartrate, which did not alter the effects of anions, generally lessened cation interference. 4. With the modified reagent and under the conditions described quantities of ammonia‐nitrogen in the range 0–100 μg. may be determined with an accuracy of 2–3%.
The use of boric acid for trapping ammonia in the micro-Kjeldahl determination is examined. A suitable procedure, together with a modified type of micro-distillation apparatus. is described.The buffer capacity of boric acid solutions was examined in relation to the sensitivity in the titration of ammonia dissolved in them. Two grades of boric acid, ' pure ' (technical) and AnalaR, were compared ; the former showed appreciable buffering on the acid side.The p H values of solutions of both grades de.creased with increasing concentration. The sensitivity of the three indicators examined for the titration of ammonia diminished with increasing concentration of boric acid as a result of the increased buffering. With concentrated boric acid, all three indicators showed wide transitional periods at the endpoint, but in solutions of concentration up to 1.0% of AnalaR grade the end-points shown by all three indicators were sharp. The methylene blue-methyl red indicator, however, was generally preferable.The ammonia-fixing capacity of boric acid was studied-in air-bubbling tests and by recoveryof a known amount of ammonia by distillation. A volume of 10 ml. of I.O?& boric acid fixed 5 mg. of nitrogen as ammonia sufficiently firmly to afford accurate analyses. For macro-determinations, IOO ml. of 2.0% boric acid held firmly up to go mg. of nitrogen.
Immobilization of nitrogen has been studied in soils treated with starch and sucrose. The conversion of inorganic nitrogen into organic form reached its maximum on incubation at 23.5' for approximately two days. The organisms concerned in the immobilization of nitrogen showed a marked preference for ammonia rather than nitrate, although some nitrate was invariably assimilated at the higher levels of added carbon compounds. The assimilation of ammonia lowered the pH of the soil, whereas assimilation of nitrate increased i t markedly.
not only be obtnined a t a very low price but also will girc high con~~ersio~~s atid yields without the need for complicntcd plnnt or process.The modern forninlin plniit is comparatircly simple in general arrangenmit niid the clieiiiicnl engineering probleins diicli it presents nre mostly concerned with corrosion. Forninlrleltyde itself is of course clieniically very active, but most of tlic corrosion is cnused by trnces of forriiic i d wliicli is inevitnldy present. Modern spccificntions cletiinntl tlint Iicnvy iiictnl contnininatioti slioultl 1)eretluccct to ii niinimuni nntl it is usunl to espect less tlriin 5 pnrts pcr iiiillioti of copper nnd 3 pnrts per million of iroii, most comnicrcial solutions being well below tltcse limits. It is tliercfore iniprncticnlde to use iron, copper, or lcntl for vessels niid plnnt in contnct nitli formnldeliy.rlc nltliougli copper cnii he ciiiploycd siicccssfully if tinned (i.e., hot-tinned, not sprnyed). Moreover copper even when utitrcntctl is not attncketl if the solution is rnnirttninct~ n b 0 W 70°, nt nliicli teniperiiture the reducing nctioii of the forninlcleliytle is such that nny dissolved iiictnl is prccipitntecl. Stainless irons and steels withstnucl forninlcleliydc in the cold satisfactorily, though sonic arc iiiucli bctter thri others in this respect. Aluminium is uscd for both plant nurf stornge or trnnsport rcssels Init is not free from nttnck, wliicli rnpidly tnkes the form of deep pitting in ordinnry coiiitiiercinl grades. Only the purest obtainnblc (99*8-99.7y0) sliould be used for formnldeliyde vessels and tliick Innterinl (4. in. to -, ' j c in.) ltns n life far longer iii proportion tliiui thin plntc nnd is therefore ultimntely iiiucli inore econgniical. Aluminium, however, is not so good for pipe-lines or wlrerc erosion can nccelerntc tlie corrosion, uiid tinned copper pipes generally give iiiost sntisfnctory service.For stornge vessels there is the clioicc of stninless steel, glnss-lined steel, rubber-lined vessels, or tnnks lined with enamels Iinring u syntlietic rcsiii I)nsc. Wooden rats have been eniployed but owing to the strong slirinking nctiori of forninldchycle on tlic wootl, hkngcs are n regular occurreticc for iiiotitlis lifter installation niid it is gencrnlly iyxessary to endure this for a year or so nnd tlteii reconstruct tlic rcssel after nll shrinkage lins taken place ; it will then giro good service. Probably the most sutisfnctory material for bulk storage vessels (to hold 50 tons or more) is reinforced concrete. The tnitk is fnbricutcd in thc satiic ntnniier as those for wnter storage nnd afterv;ards Iined with asphalt arid ncid-resisting bricks to protect tlic ceiiieiit froni nttnck by tlie foruialdehyde or forniic acid. A clteaper but less satisfnctory lining is produced by treutiiig tho concrete, after tlioroughly drying out, with hot pnraffin wns, which is nfterwards melted iiito the surfnce Ly the careful npplication of a blow lamp. Such liitiiig will lnst a coiisitlerable time but eientuully breaks down owing to the forwutioii of...
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